1. Field of the Invention
The present invention relates to a coordinate input apparatus and, more particularly, to a coordinate input apparatus for determining the coordinates of a point indicated by a pen.
2. Related Background Art
In a conventional coordinate input apparatus having a vibration input pen using ultrasonic waves as an input device, a delay time taken for vibrations caused by the vibration input pen to reach a vibration detection point is counted, and a distance is calculated by a controller on the basis of the delay time. Coordinates are then calculated and output on the basis of the distance. The size of a vibration transfer plate capable of inputting a vibration with the vibration input pen is decided by the detection gain at the vibration detection point or the output level at the vibration detection point. In contrast to this, the gain at the vibration detection point or the output level of the vibration input pen is sometimes changed in accordance with the size of the vibration transfer plate. The number of vibration detection points is set in consideration of coordinate calculation precision. For example, three or four points are arranged on an A4 size plate or the like.
When a vibration transfer plate larger than an A4 size plate is to be used, three or more vibration detection points are generally arranged in the longitudinal direction of the plate, and three or more points in its widthwise direction.
FIG. 9 is a view for explaining a conventional technique. The coordinate input apparatus shown in FIG. 9 has a vibration transfer plate and eight vibration sensors which are arranged on the plate at the positions in FIG. 9. A vibration input pen is used to indicate a position in one of areas 1 to 4. Different vibration sensors are used in the respective areas. Sensors 6a, 6b, and 6d are used in area 1; the sensor 6b and sensors 6c and 6e, in area 2; the sensor 6d and sensors 6f and 6g, in area 3; and the sensors 6e and 6g and a sensor 6h, in area 4. Switching of the vibration sensors in the respective areas is executed by a controller.
In the above conventional technique, however, since the different sensors are used to calculate coordinates in the respective areas, coordinate calculation errors are caused owing to variations in the vibration sensors constituting a coordinate input apparatus, the respective vibration detectors, and the like, the position precision of each vibration sensor, and the like.
As shown in FIG. 9, therefore, when letters "A", "B", and "C" are to be drawn, the character portions are shifted from each other at the boundaries between the respective areas, resulting in poor readability.